This application is related to U.S. application Ser. No. 10/467,436, Titled: Method of Plating Metal Leafs and Metal Membranes, by Jonah ERLEBACHER,et al., filed on Aug. 26. 2003, which is incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to methods for forming porous structures, and in particular to de-alloying metal leaf to form nanoporous metal membranes.
2. Background of the Related Art
It is known that it is possible to de-alloy a metal alloy by removing a component of the metal alloy through chemical or chemical/electrical means. For example, ancient metal gilders used de-alloying to reduce the cost of gold gilding through a method known as depletion gilding. Depletion gilding was used to selectively remove the non-gold element from a gold alloy statue to leave a thin layer of pure gold on the statue""s surface. Such a method allowed the metal gilder to use a less expensive metal of which gold was one component, yet create a nearly pure gold surface.
One example of de-alloying silver-gold alloys is disclosed in U.S. Pat. No. 4,977,038 to Sieradzki et al (hereinafter xe2x80x9cSieradzkixe2x80x9d). Sieradzki teaches a method of electrolytic de-alloying. Starting with a gold-silver metal alloy, Sieradzki teaches removing the silver by electrochemical de-alloying to leave a gold structure.
According to Sieradzki, the gold-silver metal alloy is placed in an electrolytic bath which has a high solubility for silver. By applying an electric potential between the metal alloy and a counterelectrode immersed elsewhere in the bath, the silver is selectively dissolved out while leaving the more noble gold. Sieradzki further teaches that the mechanical rigidity of the gold structure can be enhanced by heat annealing after de-alloying.
The above reference is incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Another object of the invention is to provide porous membranes and methods to create porous membranes.
Another object of the invention is to provide a method to create nanoporous metal membranes.
Another object of the invention is to provide a method to create nanoporous gold membranes.
Another object of the invention is to provide a method to create nanoporous membranes from a metal alloy.
Another object of the invention is to provide a method to create a nanoporous membrane from a gold-silver metal alloy.
In order to achieve at least the above objects in whole or in part, there is provided a method for selectively removing a component of an alloy by suspending the alloy on the surface of a de-alloying medium configured to dissolve the component.
To further achieve at least the above objects in whole or in part, there is provided a method to fabricate free-standing nanoporous metal membranes with thicknesses on the order of 100 nanometers, and porosity characterized by ligaments on the order of 10 nm diameter spaced 10 nm apart, with a surface area of approximately 10 m2 per gram of nanoporous metal membrane. The membranes are made by selective dissolution of silver from gold-silver metal leaf by supporting the leaf on nitric acid, with the acid serving to dissolve silver from the membrane. Such membranes have extremely high surface areas, and are readily attached (gilded) to many different surfaces, including metals, polymers, and ceramics.
To further achieve at least the above objects in whole or in part, there is provided a method for fabricating a free-standing nanoporous metal membrane which begins with a thin metal leaf with a composition of 50% silver and 50% gold by weight. The metal leaf may be initially supported by tissue paper. Typical thicknesses of the metal leaf include a range of about 100-500 nm, although the particular thickness may vary. The metal leaf may be transferred from the tissue paper to a glass slide by first wetting the slide and then pressing the slide to the metal leaf at which point the tissue paper may be separated from the metal leaf. Upon submersion of the glass slide into a bath of de-alloying medium, the metal leaf at least partially de-adheres from the glass slide, and the de-adhered portion is supported upon the surface of the de-alloying medium. When the de-alloying process is complete, the metal leaf can be removed from the de-alloying medium and re-adhered to the glass slide by reversing the submersion process.
To further achieve at least the above objects in whole or in part, there is provided a method for selectively removing a single metal constituent from a metal alloy by supporting the metal alloy on the surface of a de-alloying medium configured to dissolve the single metal constituent.
To further achieve at least the above objects in whole or in part, there is provided a method for selectively removing a single metal constituent from a metal alloy by supporting the metal alloy on the surface of a de-alloying medium configured to dissolve the single metal constituent, and passing an electrical current between the metal alloy through the de-alloying medium to a counterelectrode immersed elsewhere in the de-alloying medium.
To further achieve at least the above objects in whole or in part, there is a provided a method for selectively removing the silver from a gold-silver alloy by supporting a thin leaf of the gold-silver alloy on the surface of a de-alloying medium configured to selectively dissolve silver.
To further achieve at least the above objects in whole or in part, there is provided a method for making a nanoporous membrane by selectively removing the silver from a gold-silver alloy by supporting a thin leaf of the gold-silver alloy on the surface of a de-alloying medium configured to selectively dissolve silver.
A first embodiment of the present invention is directed to a method for forming a nanoporous metal membrane, including providing metal leaf comprising first and second metals supported on a substrate, and contacting at least a portion of said metal leaf with a de-alloying medium for a time effective to dissolve at least a portion of said one of said first and second metals thereby forming pores in said metal leaf, wherein said at least a portion of said metal leaf is freely supported on said de-alloying medium.
A second embodiment of the present invention is directed to a method of forming a free-standing nanoporous metal membrane, including adhering metal leaf comprising first and second metals to a substrate, de-adhering a portion of said metal leaf from said substrate, and contacting said de-adhered portion of said metal leaf with a de-alloying medium for a time effective to form pores in said metal leaf.
A third embodiment of the present invention is directed to a method for forming a nanoporous metal membrane including adhering metal leaf comprising first and second metals onto the surface of a substrate, contacting said metal leaf and said substrate with a de-alloying medium such that at least a portion of said metal leaf is de-adhered from said substrate and said de-adhered portion is freely supported on said de-alloying medium, and allowing said de-adhered portion of said metal leaf to contact said de-alloying medium for a time effective to dissolve at least a portion of said metal leaf.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.